A rotary press for making tablets of the mentioned type has become known from GB 705 000A1. From U.S. Pat. No. 5,145,693 it has also become known to measure the compression force exerted on the tablets during the production in the machine, and to single out the bad-tablets, which do not fulfill the desired requirements. It is to be understood that other parameters can be monitored as well in order to separate good tablets from bad tablets.
From EP 1 247 640 it has also become known to sort out bad-tablets by means of a nozzle. A control unit gets a bad-signal from the monitoring device and opens the nozzle, which is connected to a source of compressed air, for a short period of time, so that a bad-tablet can be directed into a bad-channel. Such an air blast is generated only within a few milliseconds, and it is naturally required to generate this air blast at a point in time in which the bad-tablet is disposed in front of the entry of the bad-channel. Moreover, the ejection of bad-tablets by means of an air blast has become known from DE 36 399 18 A1.
After the pressing process, the tablets are ejected towards the upper side of the die plate by means of the lower punches, so that a stationary wiper, in form of a wiper sheet, can pick up the tablets and direct them into a good-channel. It is to be understood, that the bad-tablets have to be sorted out before they reach the good-channel.
It is a prerequisite for a safe sorting-out of the bad-tablet, that the tablet is disposed in a stable position in front of the nozzle, because—as mentioned before—the blasting process occurs only within a few milliseconds. As a result of different characteristics of the material of the tablets and of different shapes of the tablets, tablets may queue up in the region of the tablet wiper or in the good-channel, so that the position of the tablets in front of the ejection nozzle may be altered. Thereby, a safe sorting-out process is not achieved. A congestion of tablets may even cause damage of parts of the tablet press.
It may further happen that bad-tablets blown into the bad-channel hit the wall of the bad-channel and get reflected, thereby getting into the good-channel or at least impeding the successive tablets in being conveyed. The trajectories of the blown and reflected tablets are determined to the largest extent by the air pressure, the shape, weight and thickness of the tablets.
The invention is based on the object to provide a method for sorting out bad-tablets in a rotary press for making tablets, which allows for higher reliability in sorting out the bad-tablets, without negatively affecting the conveying of the good-tablets into the good-channel.